1. Field of the Invention
This invention relates to inertial guidance and navigation systems and, more particularly, relates to a strapped down heading reference system for use with vehicles having limited pitch and roll altitudes such as land vehicles to provide accurate indications of vehicle position relative to a fixed reference axis.
2. Description of the Prior Art
Inertial guidance/navigation systems are commonly employed in both aircraft and space vehicles. These systems often comprise highly complex electronic circuitry and are substantially expensive, though not relative to the vehicles on which they are used. One particular type of guidance/navigation system is often referred to as a "heading reference system" and provides an indication of the direction toward which the aircraft is moving or "pointed," relative to a reference line, such as "true north."
When these systems are specifically adapted for aircraft guidance and navigation, they commonly employ gyroscopic rate sensors to detect vehicle movement, accelerometers to detect the rate of change of aircraft velocity and alignment, and electronic processing circuitry for computing heading indications from the various output signals of the gyroscopic and accelerometer units. One example of a heading reference system is described in the U.S. patent to Lerman et al. U.S. Pat. No. 3,281,581, issued Oct. 25, 1966. The Lerman et al. patent describes a gyrocompassing system suitable for use in aircraft applications and employs an inertial platform arrangement comprising three gyros and two accelerometers with an associated processing unit for computing various parameters of the reference system.
Heading reference systems having single-degree-of-freedom (SDF) and two-degree-of-freedom (TDF) rate sensing gyroscopes strapped down to the aircraft vehicle are also being utilized, especially since the development of lower cost two-axis rate gyros and small, lightweight digital computing apparatus having substantial processing capabilities. The U.S. Pat. No. 4,121,443 to Duncan et al., issued July 15, 1980, describes a strapped down attitude and heading reference system employing a skewed axis arrangement of two TDF rate gyros with three accelerometers applying input signals to a digital computer.
Heading reference systems are also used in land-based vehicles. For example, in the U.S. Pat. No. 4,060,718 to Huddle, issued Nov. 29, 1977, a system employing a heading reference arrangement is adapted for use in a geodetic survey method whereby earth density differentiations are indicated by deflections of a gravity vector from its expected modeled position. The reference system includes gyroscopes, a set of three mutually orthogonal accelerometers and a data processing unit for calculating gravity vector deflections. The Huddle system is not adapted specifically to vehicle guidance or navigation functions.
Heading reference systems for land-based vehicle guidance are becoming significantly important. For example, with respect to military vehicles such as armored tanks, the recent developments in electronic technology for weapons have mandated the necessity of accurate vehicle reference systems capable of use in severe environments. The design of heading reference systems for land-based vehicles is limited by several constraints. For example, such systems must be small to minimize space requirements. Additionally, in view of the relative number of vehicles on which the reference systems are installed, their cost must also be minimized. Other requirements of such systems include accuracy on unlevel terrain, simplicity of use by vehicle crew members, substantial reliability in a severe vibrational environment, and an absence of magnetic materials so that neither electromagnetic countermeasures by opposing military forces nor metallic materials of other vehicle components can interfere with the system operations. Still further, such heading reference systems should be autonomous, i.e. operate independently of other vehicle systems, and the systems should be entirely self-contained within the vehicle. If possible, the reference systems should also be portable and easy to install in the vehicle so that such systems can be readily replaced or changed to another vehicle by the vehicle crew in the field without requiring any complex calibrations. Finally, the reference systems should minimize any use of radio communications for purposes of security.
Many military armored vehicles presently use a hand-held magnetic compass system as the primary means of determining vehicle heading. Such compass systems require a vehicle crew member to dismount from the vehicle and position himself far enough from the vehicle so as not to have the magnetic components of the vehicle produce an erroneous reading. Clearly, such procedures are unacceptable in terms of safety and are essentially impossible to perform during the night or in conditions of poor visibility.